Effect of microstructure variation on differential hardening behavior of steel sheets under biaxial tensile state

Abstract

For further improvement of press formability of steel sheets, it is important to clarify the relation between macroscopic mechanical properties and microstructure under multi-axial deformation state. The objective of this work is to correlate the work hardening behavior with microstructure evolution under biaxial tensile state. The materials used in this study were interstitial-free (IF) steels with different grain sizes. First, the work hardening behaviors were examined by conventional uniaxial tensile and bulge tests, and the differential hardening behavior was measured. Next, in-situ observation of microstructure evolution was conducted for uniaxial and biaxial tensile deformation using the microscopic biaxial tensile system with electron back scatter diffraction patterns (EBSD) with scanning electron microscope (SEM) analysis. The texture development and inhomogeneous deformation were observed clearly under the biaxial tensile state. The effect of texture development on the yield locus was investigated using Taylor-Bishop-Hill (TBH) theory and the orientation distribution function (ODF). It was found that the differential hardening at the grain level was caused by the crystal rotation. Finally, the inhomogeneous deformation was analyzed. The material with fine grain exhibited relatively homogeneous deformation and high work hardening ratio even over a large strain range under biaxial tensile state. It was suggested that the texture development over a large strain range and the homogeneity of deformation resulted in the differential hardening behavior.